1. Field of the Invention
The invention in general relates to optical radar systems and more particularly, to the generation of an optical beam therefor.
2. Description of Related Art
A system which transmits and receives electromagnetic radiation at optical frequencies is known by various names such as LIDAR, LADAR, laser radar and optical radar. The source of radiation in an optical radar system is a laser which transmits radiation generally in the low to high THz (terahertz) region.
A typical RF radar used for air-to-ground operations might transmit a waveform with a 1 GHz (gigahertz) bandwidth at an average power level of around 100 watts. An optical radar offers significant improvement in bandwidth because of its much higher frequency of operation in the high THz region.
In the operation of a typical radar, RF or optical, a narrow pulse provides a higher resolution than a wide pulse, however a wide pulse provides for higher energy and therefore a greater range. These two inconsistencies are resolved by a technique known as pulse compression. That is, a wide pulse which has a predetermined modulation is transmitted and the return pulse is compressed to achieve the resolution desired.
In an optical radar such modulation of higher frequencies is achieved more readily over the lower frequencies, however such higher frequencies are not amenable to long range transmissions because of absorption by water vapor, carbon dioxide and particulate matter in the atmosphere. The lower frequencies are not affected as much by these airborne contaminates. The present invention provides for an optical radar system which modulates high frequency optical signals and transmits an output signal which combines a plurality of lower frequency optical signals, resulting in a high power output signal substantially unaffected by atmospheric contamination.